Computer technology continues to expand at an enormous rate and the level of the expertise of users is expected to decline at a similar rate. In order to allow these inexpert operators to interface properly with sophisticated computer systems, numerous devices have been developed to eliminate the need for complete dependency upon the use of a keyboard for entry of information.
One means of facilitating the interaction between the operator and the computer is a touch sensitive panel overlaying the display screen of the computer. The touch sensitive panel is designed so that a touch contact applied at a particular location causes the generation of a meaningful signal to a software controlled microprocessor. Data presented on the face of the screen provides options to the operator or asks various questions. Possible alternatives or responses to the questions are also provided on the screen. By contacting the touch sensitive panel at the location where the desired response or answer appears upon the screen, the desired response is sent to the microprocessor to update the display, perform a desired operation, or provide new options for the operator to respond to.
In the past, computers have typically utilized a cathode ray tube (CRT) for display purposes because of cost. For example, a touch sensitive CRT display system is disclosed in applicant's co-pending application, Ser. No. 448,947, filed Dec. 13, 1982, now U.S. Pat. 4,567,430. In this application, the raster scan system of a CRT is used in conjunction with a photovoltaic overlay to allow position detection of a touch relative to images on the display.
Another example of a touch sensitive system used with CRTs is disclosed in U.S. Pat. No. 4,423,299 granted Dec. 27, 1983 to M. Gurol, et al. In this system, an orthogonal arrangement of transparent strips are used to provide switches for locating positions on a CRT display. Essentially, one set of strips is placed on the display and another set at right angles is placed on a flexible membrane which is separated from the display. By touching the flexible membrane to the display, contact is made between the conductive strips to close a circuit which indicates the location of the touch approximately relative to the images on the display.
More recently, so called "flat panel display" technologies have been developed which provide an alternative to the conventional CRT. These flat panel displays are smaller, lighter, and more compact than a conventional CRT. Some flat panel displays have a depth of less than one and one-half inches. In addition, many flat panel displays require much lower power for their operation that a CRT. Currently, they are more expensive than CRT's, but the price is expected to be lower with advances in technology. Flat panel displays generally fall into two categories of "active" and "passive" types.
The active type displays include light emitting diode (LED) displays, various types of plasma displays, vacuum fluorescent displays (VFDs), and thin film electroluminescent (TF-EL) displays. Recently, flat cathode ray tubes and electrophoretic displays have been developed and added to this category.
The passive type displays include liquid crystal displays (LCDs) and electrochromic displays.
Essentially, these displays consist of a pair of electrodes having a display medium disposed therebetween which is responsive to electric fields imposed on the electrodes. The active displays have materials or gasses which emit light under the stimulus of the field. The passive displays generally reflect or transmit light depending on the electric fields and do not in and of themselves emit light.
One example of an active display is the gas plasma display. The gas plasma display utilizes two panels of glass to enclose a gaseous mixture consisting principally of neon. An electrode is disposed on each of the glass panels and the application of an AC or DC voltage across the electrodes causes the gas to ionize and emit light.
The liquid crystal display is typical of the passive type display. In a liquid crystal display, two panels of glass enclose a liquid having crystaline properties. When a DC voltage or a multiplexed AC voltage is applied across the liquid crystal, the liquid crystal orients itself from its normal transparent state, which allows the reflection of ambient light from a reflective surface on the back glass plate, to an opaque state.
To provide alphanumeric or graphic information on a flat panel display, a grid of electrodes is generally used on either side of the display material. In a display having a picture, each pair of electrodes is known as a pixel and is generally located in an X-Y orthogonal matrix. To cause the pixels to provide a display by either emitting light or changing reflectivity, a drive electronics technique known as matrix addressing is used. In matrix addressing, each pixel is related to a set of coordinates in the matrix and has a row and column address. Each of the pixels has one terminal connected together with other pixels in a row, and the other terminal connected with other pixels in a column located orthogonally to the row. When a pixel is selected, a voltage is applied to its corresponding row and column leads to define the coordinates of the selected pixel. In order to avoid activating invalid pixels, the row and column leads are multiplexed so only one row lead and one column lead receives an electrical signal (addressed) at a given instant in time.
The matrix addressing of large displays requires a very high rate of multiplexing which results in signal pulses having very short duty cycles which may be insufficient to cause a response in the display medium. In addition, some display medium, such as liquid crystal materials, have reduced visual performance using the short pulses. In order to overcome these problems, a transistor or diode switch in combination with a storage capacitor is incorporated at each pixel matrix junction. The capacitor, charged by the transistor provides a sufficiently long pulse across the display medium to cause a visual response.
Touch sensitive display systems have been developed for flat panels, but these are generally combinations of flat panel technology with previously existing capacitive touch switch technology. In an article by P. W. Alexander and D. A. Levien, DCEL Displays With Integrated Fixed Legend and Dot Matrix Touch Switches, SID 84 DIGEST, pp. 189-191 (1984), there is disclosed a capacitive touch switch overlay for a DC electroluminescent display in which a matrix array of display cathodes and display anodes are used to provide an excitation across a luminescent medium. The switch sensing is achieved by pulsing the anode driver enable line to cause all anodes to be driven simultaneously.
Heretofore, there has been developed no touch sensitive system which could be easily and economically utilized with both active and passive displays.